Atherosclerosis is a condition in which fatty deposits called plaque collect along the walls of arteries. The accumulation of plaque can erode the wall of the artery, diminish its elasticity (stretchiness), and interfere with blood flow. A rupture of the plaque can cause the formation of a blood clot that blocks the flow of blood through the artery, leading a heart attack or a stroke. Clots can also form around the plaque deposits, further interfering with blood flow and posing added danger if they break off and travel to the heart, lungs, or brain.
Clinical studies of atherosclerosis often rely on angiography to produce an image of the luminal border of a blood vessel of interest and intravascular ultrasound (IVUS) to obtain images of the plaque and the luminal wall from an ultrasonic transducer inserted within the blood vessel. Although IVUS provides detailed images of the lumen, vessel wall, and plaque, it yields no information about the specific location or absolute orientation of the image. Current techniques for determining the absolute orientation of an IVUS image take advantage of the irregularities of the lumen (i.e., bumps and crevices) that appear in both the IVUS image and the angiogram. With these techniques, the IVUS image is rotated relative to the angiogram until a correlation between the irregularities in both the angiogram and the IVUS image reaches a maximum. The matching of the irregularities may be inexact due in part to ambiguities in the images and to some of the irregularities changing over time.